Unlike the features shown in the above two images, many systems on Mars
do not show evidence of catastrophic flooding. Instead, they show a
resemblance to drainage systems on Earth, where water acts at
slow rates over long periods of time. As on Earth, the channels shown
here merge together to form larger channels.

However, these valley networks are less developed than typical terrestrial
drainage systems, with the Martian examples lacking small-scale streams
feeding into the larger valleys. Because of the absence of small-scale
streams in the Martian valley networks, it is thought that the valleys
were carved primarily by ground water flow rather than by runoff of
rain. Although liquid water is currently unstable on the surface of
Mars, theoretical studies indicate that flowing groundwater might be
able to form valley networks if the water flowed beneath a protective
cover of ice. Alternatively, because the valley networks are confined
to relatively old regions of Mars, their presence may indicate that Mars
once possessed a warmer and wetter climate in its early history.
(Image Credit: Calvin J. Hamilton; Caption: LPI)